The present invention relates to MLS-type landing systems and more specifically to a process for increasing the range and the protection against jamming of such a system, as well as to means for performing this process.
The microwave landing system or MLS makes it possible to assist an aircraft when landing by supplying it with different information called functions, such as its azimuth angle relative to the axis of the runway, its site angle relative to the horizontal and optionally other ancillary functions, such as e.g. the rear azimuth and data, comprising so-called basic data and so-called auxiliary data. These different information are permanently transmitted from the ground in time multiplexing on the same frequency in accordance with characteristics standardized by the International Civil Aviation Organisation (ICAO), appendix 10, paragraph 3.II and are decoded by each aircraft in question.
Each of the aforementioned functions is broken down into two successively transmitted parts. The first is a preamble, which serves to supply the aircraft with an identification of the following function. This preamble is transmitted by a sector antenna, i.e. a fixed antenna transmitting throughout the entire area or sector to be covered by the MLS. According to the ICAO standard, the preamble is in the form of a twelve bit word, more particularly comprising seven bits constituting an identification code designating in a biunivocal manner functions transmitted in two-state (0,.pi.) differential phase shift keying (DPSK) modulation. The actual function is then transmitted and in the case where this function is data, it is transmitted by the sector antenna in two-state DPSK modulation. When this function is an angular information, it is constituted by two pulses transmitted then with the aid of a scanning antenna, in accordance with the so-called time reference beating or beat beam described hereinafter with references to FIGS.1a, 1b and 2.
FIG.1a illustrates the azimuth angle encoding principle. In an azimuth station are transmitted two different radiations by two separate antennas which, for simplification purposes, are shown at the same point A.sub.Z. Thus, starting from the latter, there is the transmission diagram of the preamble, designated P.sub.AZ, transmitted by the sector antenna in the complete coverage zone of the MLS and which is represented in the drawing by an angle .alpha.. Starting from A.sub.Z, there is further the diagram of a flat, vertical beam B.sub.AZ, called the beat beam, transmitted by an electronic scanning antenna. At constant speed, beam B.sub.AZ performs an outward scan and then, after a stop time, a return scan. This takes place in a scanning zone forming an angle .beta..sub.Z in the drawing, which can be equal to or less than the aforementioned coverage angle .alpha.. In this drawing, .beta..sub.Z is shown smaller than .alpha.. In addition, arrows A.sub.Z and R.sub.Z show the outward and return scan paths of beam B.sub.AZ in the scanning zone .sym..sub.Z. It is also possible to see an aircraft A.sub.V, which is not e.g. correctly aligned with the runway axis ZZ.
In accordance with ICAO standards, angle .sym..sub.Z is equal either to 20.degree., being broken down with respect to axis of runway ZZ into two semi-angles -.sigma..sub.M =+.sigma..sub.M =10.degree., or 80.degree. with -.sigma..sub.M =+.sigma..sub.M =40.degree.. The width or aperture of the beam B.sub.AZ is 1.degree. to 4.degree. in the plane of the drawing and 7.5.degree. to 14.degree. in the vertical plane.
FIG. 2 illustrates the operation of this device. The transmissions effected from the ground installations are shown as a function of time on an upper line. Thus, said installation transmits preamble P.sub.AZ, followed by the outward scan time A.sub.Z of the beat beam B.sub.AZ, then the return scan time R.sub.Z of the same beam. F.sub.AZ is the azimuth function finish time.
On the lower line of the diagram of FIG. 2 is shown the signals received on board the aircraft A.sub.V, while ignoring the signal propagation time. These signals are firstly the preamble P.sub.AZ in principle identical to the transmitted preamble, and a pulse at each of the instants (output and return) where aircraft A.sub.V is illuminated by the beam B.sub.AZ from the ground antenna. These two pulses are respectively designated A.sub.Z1 and A.sub.Z2 and are separated by a time interval .DELTA. t.
Apart from its identification function, the preamble also has the function of supplying a time reference. The preamble has a part which precedes the identification code and uses a special code, called the BARKER code with five bits (11101) and the time reference (designated t.sub.REF) is taken at the transition instant 0-1. This time reference makes it possible to decode the information in the aircraft receiver. In the case of an angular function, the center time (t.sub.o) between the outward and return scans has, according to the ICAO standards, a fixed time position relative to instant t.sub.REF (t.sub.o -t.sub.REF =T.sub.M) and this applies throughout the zone covered by the system. This facilitates decoding and makes it possible to validate the angle information. In the case of a data function, the instant t.sub.REF makes it possible to correctly decode the data bits.
The measurement in the aircraft receiver of the time which has elapsed between the peaks of pulses A.sub.Z1 and A.sub.Z2 supplies the value of the azimuth angle .theta. where the aircraft is located by means of the relation: ##EQU1## in which: T.sub.o is the time which has elapsed between two pulses when the receiver is located in azimuth 0.degree. (on the axis of runway ZZ), this time being deduced from the ICAO standards of the MLS system;
V the ground antenna scanning speed, which is also standardized and which corresponds to an outward - return scan repetition rate of approximately 13 or 39 times per second, as a function of the type of MLS station.
FIG. 2 also shows two straight lines supplying the correspondence between the aircraft azimuth angle .theta. and the transmission and reception time of the various signals, the maximum scan angles -.theta..sub.M and +.theta..sub.M of the beam B.sub.AZ being illustrated.
In the same way in which this has been done in FIG. 1a, FIG. 1b shows the site transmission principle. FIG. 1b shows the site station S from which are transmitted two beams by two separate antennas, namely a sector antenna transmitting the site preamble P.sub.S, whose diagram is shown in FIG. 1b, and the other transmitting a flat beat beam B.sub.S scanning the scanning zone of angle .beta..sub.S, which is performed in the same way as for the azimuth beat beam B.sub.AZ. According to ICAO standards, the angular width of the beat beam B.sub.S in the plane of the drawings is between 1.degree. and 2.degree., while being 80.degree. in the perpendicular plane.
In such an MLS station, the present invention proposes to overcome the problem of increasing the range of the system, without increasing the transmitting power, as well as that of possible jamming.
As can be gathered from the above description, the jamming of e.g. an angular function can take place either at the preamble by jamming the transmission of the sector antenna in question, or on the transmission of the beat beam supplying the actual angular information, or on both of them. The analysis of the respective transmission levels of the preamble and the beat beam shows that the gain of the scanning antenna exceeds that of the sector antenna by a theoretical value between 10 and 20 dB. In practice, the gains of the scanning antennas are not as high, due to the fact that the construction of such antennas is more complex and leads to higher losses, but still remains above that of a sector antenna. Moreover, the coverage, in the perpendicular direction to the scan, of the azimuth beat beam exceeds, according to ICAO standards, the width of the site beat beam, so that the scanning antenna used for the azimuth function has a higher gain. Thus, the part of the MLS signal which is most sensitive to jamming, because it corresponds to the lowest level, is the preamble radiated by the sector antenna.
In order to reduce the vunerabiity of the MLS signal to jamming, it is known to increase the transmitting power of the preamble. However, this function suffers from the following disadvantages. Its cost: due to the power and frequency levels used at present, the sector antenna transmitter is constructed with the aid of transistors and any significant increase in this transmission power would make it necessary to use more complicated and costly travelling wave tubes. The reduction in the discretion of the system: an increase in the transmission power increases the range and consequently the vulnerability to possible countermeasures. Its very relative efficiency: a corresponding increase in the power of the jammer would lead to the same problem again.
It is also known to reinforce the preamble with the aid of a scanning antenna successively pointed on the different angles of the coverage. This system is more particularly described in the THOMSON-CSF French Patent No. 2 519 430. However, in this system, the receiver only decodes the reinforced preambles when the scanning antenna is pointed on it, while in the intervals it is not guided, which is a disadvantage.
Moreover, in the absence of jamming, the range of an MLS station is defined by the range of the preamble radiated by the sector antenna, to the extent that it is the lowest signal. Thus, a reinforcement of the preamble makes it possible to increase the range.